'Spiraling' (sometimes spelled 'spiralling' in British English) describes losing control of one's thoughts or emotions, going into a downward loop of anxiety, panic or self-criticism. 'I'm spiraling' = 'I'm overwhelmed with anxious thoughts'. If your teen uses the word, they are naming a mental-health experience — generally a healthy sign of self-awareness. Worth checking in gently to see what is driving it and whether they have coping strategies or support.